Binders can be added to catalytic materials to form an aggregate catalyst with modified properties, such as improved physical properties. One type of catalyst that can be modified with a binder material is a molecular sieve type catalyst such as zeolite.
Zeolite is a crystalline alumino-silicate that is well known for its utility in several applications. It has been used in dealkylation, transalkylation, isomerization, cracking, disproportionation, and dewaxing processes, among others. Its well-ordered structure is composed of tetrahedral AlO4−5 and SiO4−4 molecules bound by oxygen atoms that form a system of pores typically on the order of 3 Å to 10 Å in diameter. These pores create a high internal surface area and allow the zeolite to selectively adsorb certain molecules while excluding others, based on the shape and size of the molecules. Thus, zeolite can be categorized as a molecular sieve. Zeolite can also be termed a “shape selective catalyst.” The small pores can restrict reactions to certain transition states or certain products, preventing shapes that do not fit the contours or dimensions of the pores.
The pores in zeolite are generally occupied by water molecules and cations. Cations balance out the negative charge caused by trivalent aluminum cations which are coordinated tetrahedrally by oxygen anions. Zeolite can exchange its native cations for other cations; one example is the exchange of sodium ions for ammonium ions. In some ion-exchanged forms, such as the hydrogen form of zeolite, the catalyst is strongly acidic. The acidic active sites are useful for alkylation as well as many other reactions. For instance, zeolite can serve as a solid acid catalyst for Friedel-Crafts alkylations, replacing traditional aluminum trichloride and other liquid acid catalysts that can be corrosive and damaging to the reactor.
One alkylation reaction for which zeolite can be used as a solid acid catalyst is the alkylation of benzene with ethylene to form ethylbenzene. Ethylbenzene is an aromatic hydrocarbon with the chemical formula C6H5CH2CH3; it consists of a six-carbon aromatic ring with a single attached ethyl group. It can undergo a dehydrogenation reaction to form the monomer styrene, the monomer from which polystyrene is made. Polystyrene is a plastic that can form many useful products, including molded products and foamed products, all of which increase the need for production of styrene's precursor, ethylbenzene.
In the ethylation of benzene, zeolite can be categorized as a heterogenous acid catalyst, because it is in a different phase than the reactants. The zeolite catalyst is solid and usually supported by an alumina or silica binder to increase its mechanical stability inside the reactor bed. The reactants, on the other hand, are either in the liquid, vapor, or supercritical phase. The production of ethyl benzene via alkylation has been done with benzene in the gaseous phase, but it is also possible to use liquid phase alkylation, which requires lower temperatures. Liquid phase alkylation can be more economical in certain situations and can decrease the production of unwanted by-products.
However, operating at the lower temperatures required for liquid phase alkylation can have the effect of increasing the catalyst's sensitivity to impurities in the feedstock. The acid sites in zeolite are prone to deactivation, especially in liquid phase reactions, by polar molecules containing nitrogen, oxygen, and sulfur functional groups. The deactivation of the catalyst's acid groups decreases catalyst efficiency. One result can be that the catalyst's deactivation rate increases, necessitating more frequent catalyst regeneration and shortening the overall lifetime of the catalyst. Catalyst regeneration and replacement can both require process shutdown of the reactors, costing time and money, and thus it is desirable to perform these functions infrequently.
One solution has been to filter polar poisons from the feedstock prior to its contact with the zeolite catalyst, such as by passing the feed stream through one or more molecular sieves prior to its entering the main reaction bed. In many cases, however, trace amounts of these polar contaminants still reach the zeolite catalyst.
Because the present technology for purifying the alkylation feedstock fails to entirely prevent small amounts of polar contaminants from entering the reaction bed, it would be desirable to inhibit the contaminants that do enter the reaction bed from contacting the active sites of the zeolite catalyst. It is desirable to inhibit contaminants from contacting the active sites of the catalyst, whether zeolite or other catalyst types.